Means for initiative synchronization and cell searching based on the CDMA system of multipath energy window

ABSTRACT

The present invention discloses an apparatus for initial synchronization and cell search in a CDMA system based on multipath energy window. The apparatus perform initial synchronization and adjacent cell search by using multipath energy window based on the processing method of multipath energy window. The apparatus can be implemented based on a parallel correlator bank. This invention also provides the application of the apparatus to a coherent spreading receiver.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/CN01/01634, filed 18 Dec. 2001 andpublished as WO 02/80425 on 10 Oct. 2002, not in English.

FIELD OF THE INVENTION

The present invention relates to a CDMA (code division multiple access)mobile cellular communication system.

BACKGROUND OF THE INVENTION

In a lot of mobile communication standards, recently, CDMA cellularcommunication technique shows great potential for its featuresassociated with large capacity, simple frequency planning, goodcommunication quality and small electromagnetic interference. CDMAmobile communication has become the key technique of next generation ofmobile communication systems.

Most of CDMA cellular mobile communication systems employ directsequence spread spectrum (DSSS) technique. A transmitter converts thenarrow band user information into wideband spread spectrum signals bymultiplying narrow band information by a spreading code (PN code). Thisprocess is referred to as spread spectrum. A receiver restores thenarrow band user information by multiplying received wideband spreadingsignals by the locally generated synchronous spreading code andintegrating operation. This process is referred to as despreadingoperation. Accordingly, how to synchronizing local spreading code withthe received signals is of importance to accomplish despreadingoperation in a receiver.

There is multipath fading in a mobile communication system, which causesserious multipath interference. In a CDMA cellular mobile communicationsystem, the bandwidth of transmitting signals is typically much largerthan that of user information because of the use of spread spectrumtechnique. Therefore, the CDMA cellular mobile communication system hasthe capability of distinguishing multipath. Therefore, it is possible tomore finely distinguish multipath. However, since the multipath signals,which can be distinguished, have large radomicity, CDMA receivers shouldhave the ability to overcome the multipath fading.

To synchronize with local spreading code in a CDMA receiver, thereceived signals need to be initially synchronized and finelysynchronized. The initial synchronization process is also referred to asPN code acquisition process. The fine synchronization process is alsoreferred to as PN code track process. The PN code initialsynchronization process make the phase difference between the local PNsequence generated by the receiver and PN spreading sequence of thereceived signals smaller than a proper threshold δ. After the process ofPN code initial synchronization, due to local oscillationfrequency-offset, channel multipath fading, and Doppler frequency-offsetetc., the timing will slide. Therefore, the received signals shall betracked in real time so that the local PN sequence can finelysynchronize with received signals all the time.

In CDMA cellular mobile communication systems, in addition to initiallysynchronize and track the cell which, a mobile terminal is located, itis necessary to search adjacent cells to judge whether or not the mobileterminal performs hand-off or macro-diversity receipt of the arrivalsignals of a plurality of base stations. The method of searchingadjacent cells is similar with that of initial synchronization to a cellexcept for that the process of searching adjacent cells is onlyperformed in partial area determined in advance without searching allphases of PN code.

CDMA systems have time restricts for initial synchronization of a celland searching adjacent cells. The time for initial synchronization andsearching adjacent cells shall be as short as possible. There are a lotof methods of initial synchronization and searching adjacent cells inDS/CDMA systems. These method may be classified into parallel initialsynchronization, series initial synchronization and, parallel and serieshybrid initial synchronization in terms of structure domain. In terms ofphysical domain, the method of initial synchronization may be classifiedinto time domain initial synchronization, frequency domain initialsynchronization. In terms of despreading methods, the method of initialsynchronization may be classified into initial synchronization based oncorrelators and initial synchronization based on matching filter. Theinitial synchronization based on correlators includes fixed correlationlength detection and variable correlation length sequential detection.

Conventional initial synchronization and search is used for a certainsingle-path signal. In multipath transmission circumstances, however,the real received signals typically comprise a plurality of multipathcomponents which have different time-delay. The results of initialsynchronization and search for a certain single-path signal aregenerally not reliable. The present invention is on the basis of thefollowing fact: the energy window of multipath signal and the barycenterposition of the window is relatively stable although the amplitude andphase of the signals for each arrival path randomly change in multipathcircumstances. If the initial synchronization phase of the local PN codeor the PN code phase of adjacent cells is determined based on maximizingmultipath signal energy window, and the PN code is finely tracked basedon barycenter change of the multipath energy window, the obtainedspreading receiver can overcome the non-determinacy due to theprocessing of single path signals, thereby improving the stability ofcoherent spreading receivers.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anapparatus for initial synchronization and adjacent cell search of a CDMAspreading receiver which is introduced a designing method of maximizingmultipath energy window with respect to the non-determinacy of the multifading signals in mobile communication circumstances, thereby improvingthe performance of the CDMA spreading receiver and reducing hardwarethereof.

The present invention provides an apparatus for initial synchronizationand cell search in CDMA systems based on multipath energy window. Theapparatus of the invention performs initial synchronization and adjacentcell search by using multipath energy windows in accordance withmultipath energy window processing method. The invention also provides aparallel implementing method based on correlator bank and describes theapplication of the method to a coherent spreading receiver.

To achieve the object of the invention, there is provided an apparatusfor initial synchronization and cell search in CDMA systems based on amultipath energy window, the apparatus comprises a plurality of tapdelay lines, which the number S of the tap delay lines is determined bythe minimal time required for initial synchronization and cell search,for sampling the baseband signals r(t) with the rate T_(c)/M, sendingthe S tap outputs from the plurality of tap delay lines to S parallelcorrelators; S parallel correlators each of which performs channelevaluation of a single path in one integrating period NT_(c), clearsrespective multiplying accumulation units after sending the evaluatedresult to respective amplitude judging units; a plurality of amplitudejudging units for judging whether the square value of the channelevaluation amplitude obtained from respective correlators is larger thana threshold, sending the evaluated amplitude which the pure interferencepaths are removed to a buffer unit, and then providing the evaluatedamplitudes to a sliding multipath energy window calculation unit one byone through a parallel-series conversion unit; a sliding multipathenergy window calculation unit for calculating S outputs from slidingmultipath energy windows for the S channel evaluations obtained in everyintegration interval; a maximal energy window calculation unit forcomparing every value output from the sliding multipath energy windowsand selecting the amplitude having maximal value and the phase of thelocal pilot PN code associated with the amplitude.

Next, the principle of the invention will be discussed.

1. Evaluating Multipath Fading Channel Parameters

A pilot channel in a CDMA system is used for transferring a pilotsequence known in advance which may provide a system timing, extractcarriers, evaluate channels, and execute hand-off, etc. The equivalentbaseband receiving signals may be expressed as shown in equation (1)when the system simultaneously transmits signals through a plurality ofchannels,

$\begin{matrix}{{r(t)} = {{\sum\limits_{n}^{\;}{c_{n} \cdot {\sum\limits_{i}^{\;}{s_{i}\left( {t - {n/W}} \right)}}}} + {z(t)}}} & (1)\end{matrix}$wherein s_(i)(t) represents the signs and equivalent baseband signalstransmitted through ith code division channel in downstream channels.The term of i=0 corresponds to the pilot channel. z(t) is complex WhiteGaussian noise of zero average value, c_(n) is a fading factor of nthpath of the channels. The purpose for evaluating channel parameter is toevaluate channel fading factor c_(n) based on the received signals r(t)and the known pilot sequence s₀(t).

It is assumed that frequency selectivity slow fading channel model isused as a mobile channel, c_(n) is then approximate to a constant withinthe channel evaluation region. The evaluation value of c_(n) is given asfollow:

$\begin{matrix}{{\overset{\_}{c}}_{n} = {{\frac{1}{{NE}_{c}}{\int_{0}^{{NT}_{c}}{{{r\left( {t - {nT}_{c}} \right)} \cdot {s_{0}^{*}(t)}}{\mathbb{d}t}}}} = {c_{n} + N_{a} + N_{c} + N_{z}}}} & (2)\end{matrix}$wherein N_(a), N_(c), and N_(z) are the outputs caused by multipathinterference, multiple access interference and white noise passedthrough a correlator due to the non-ideal correlation characteristic,T_(c) is a time width of one chip, NT_(c) is an integration region of achannel evaluation, and E_(c) is energy transmitted through a pilotchannel within one chip.

In initial synchronization stage of a CDMA receiver, the phaseinformation of received signals can not be known. It is necessary toevaluate multipath fading channels in fraction intervals, and tryevaluation using local pilot sequence (PN code) with different phase. Inthis case, following equation (4) can be derived from equation (2).

$\begin{matrix}{{{{\overset{\_}{c}}_{n,m}(k)} = {\frac{1}{{NE}_{c}}{\int_{0}^{{NT}_{c}}{{{r\left( {t - {nT}_{c} - {{mT}_{c}/M}} \right)} \cdot {s_{0}^{*}\left( {t - {{kT}_{c}/M}} \right)}}{\mathbb{d}t}}}}},{m - 0},1,{{\Lambda\; M} - 1}} & (3)\end{matrix}$wherein T_(c)/M is a fraction sampling interval, k is a certain possiblephase parameter of tile local pilot PN sequence.

2. Multipath Energy Window and Initial Synchronization Method of PN Code

The effective distribution range of channel fading factor c_(n) inequation (1) is defined as energy distribution window of multipathsignal (hereinafter is referred to as multipath energy window). The sizeof the window may be determined by time-delay extend range of multipathchannels. For the sake of simplifying discussion, the effectivedistribution range of c_(n) may be set to n∈[−L₁,L₂]. The size of thewindow in multipath fading circumstances may be set differently fordifferent areas, for example, 3 μs for cities, 6 μs for countries, and15 μs for mountain areas. The size of window is associated with thecircumstances where the cellular communication system is located, and isregardless of the used frequency band. The size of multipath energywindow may be selected according to the maximal likelihood value, forexample, no more than 30 μs, and then the value of L=L₂−L₁+1 is not morethan 30 μs/T_(c) so that a spread spectrum receiver can be used invarious circumstances.

In a multipath energy window, not all signal arrival paths areeffective. To this end, a threshold may be set to judge the signalenergy (i.e., intensity of c_(n)) for each of paths in the window. Asignal arrival path is judged as effective path when the signal energyis larger than the threshold. Otherwise, the path is judged as a pureinterference path. To avoid the degradation of the performance, thecalculation is not applied to all pure interference paths. The thresholdis set slightly larger than the side lobe value of a pilot signal (PNcode) partial correlation value.

To obtain sufficient acquisition precision, a receiver samples thereceived signals using over-sampling technique. The sampling rate is Mtimes the chip rate of PN code. Assuming the length of PN code requiredfor synchronizing is p, the PN code initial synchronization method ofthe invention selects a phase from M×P possible PN code phases, andmaximize the multipath energy contained in the multipath energy window.

$\begin{matrix}{{E_{win}(k)} = {\sum\limits_{n = {- L_{1}}}^{L_{2}}{\sum\limits_{m = 0}^{M - 1}{{{\overset{\_}{c}}_{n,m}(k)}}^{2}}}} & (4)\end{matrix}$the initial synchronization method based on multipath energy window isthen described as selection of a value k which makes following equation(5) have a maximal value from all possible values k of local PN codephase:

$\begin{matrix}{\max\limits_{k}{E_{win}(k)}} & (5)\end{matrix}$Thus, the phase of local pilot PN code, which corresponds to the energywindow having maximal value, is the best PN code phase k_(opt).

On the other hand, it can be seen from equation (3), the multipathenergy window calculation as shown in equation (4) exists followingderivative relationship associated with a sliding window:E _(win)(k+1)=E _(win)(k)−| c _(L) ₂ _(,M-1)(k)|² +| c _(L) ₁_(,0)(k+1)|²   (6)Thus, initial synchronization calculation can be greatly simplified.

3. Adjacent Cell Search Method Based on Sliding Multipath Energy Window

The method of searching adjacent cells is similar with that of PN codeinitial synchronization except for that the PN code used in theequations is a pilot signal sequence of a certain adjacent cell, and theareas which are searched are determined by a base station in advancewithout searching all phases of PN code.

4. The Processes of confirming PN Code Initial Synchronization and CellSearch

In above initial synchronization process, whether or not the result ofthe initial synchronization is valid is judged by comparing to athreshold one time. In practice, it can be judged by comparing to athreshold a plurality of times. At that time, it only need to calculatethe energy window at k=k_(opt) with a plurality of times. The initialsynchronization is successful if each of the results is larger than thethreshold. In this case, only the time taken for initial synchronizationis slightly longer. Another practical process of confirming initialsynchronization checks whether or not the receiving states of other datachannels are normal after finishing the initial synchronization so as tojudge whether the initial synchronization is successful or not.

The process of judging whether or not the result of adjacent cell searchis valid is similar with the process of PN code initial synchronization.

The advantages of the present invention are as follows:

1. The initial synchronization method based on multipath energy windowaccording to the invention can maximize the multipath time-delaydistribution energy, can overcome the non-determinacy of the system dueto the processing of single path signals in conventional initialsynchronization method. Therefore, the apparatus according to theinvention is applicable to a mobile cellular communication system in thecircumstances of multipath fading channel.

2.The apparatus of the invention employs a sliding multipath energywindow calculation method and is facilitated to implement.

3. The apparatus of the invention integrates the initial synchronizationand adjacent cell search of a CDMA receiver in the manner of timedivision multiplexing (TDM), the complexity of the hardware used in asystem is therefore greatly reduced.

4. It takes a shorter time for initial synchronization and adjacent cellsearch since the calculation is operated in parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings, which illustrate examples of thepresent invention.

FIG. 1 schematically illustrates a block diagram of implementing initialsynchronization and adjacent cell search based on the multipath energywindow of the apparatus according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, there is shown preferredembodiment of the invention. FIG. 1 shows a block diagram of theapparatus for initial synchronization and adjacent cell search based onthe multipath energy window of the apparatus according to an embodimentof the invention. The apparatus comprises S tap delay lines, S parallelcorrelators, a local cell PN code generation unit, an adjacent cell PNcode generation unit, amplitude judgment units, a buffer unit, aparallel-series converter unit, a sliding multipath energy windowcalculation unit, a maximal energy window calculation unit, and ainitial synchronization confirmation unit. The functions of each of theunits will be described as follows.

The baseband signals r(t) sampled in T_(c)/M rate is sent to the S tapdelay lines. The number S of the tap delay lines is determined by thetime taken for initial synchronization and cell search. S tap outputsare provided to the S parallel correlators respectively.

Each of the S parallel correlators (multiplying accumulation units)performs channel evaluation integration operation as shown in equation3. If it is used for initial synchronization process, the integrationoperation is performed with the PN code output from the PN codegeneration Unit for current cell. If it is used for cell search process,the integration operation is performed with the PN code output from thePN code generation unit for adjacent cell. Each of the correlatorsperforms the channel evaluation for one path in one integration periodNT_(c). The integrated results are supplied to the correspondingfollowed amplitude judgement units, and then the multiplyingaccumulation units are cleared.

The amplitude judgement units judge the amplitude square values of thechannel evaluation obtained from the correlators, send the evaluatedresults of which pure interference channels are removed to the bufferunit. Then, the evaluated results output from the buffer unit is sent tothe sliding multipath energy window calculation unit through theparallel-series converter unit.

The sliding multipath energy window calculation unit performs thecalculation as shown in equation 6. S output values of the slidingmultipath energy windows could be obtained for the S channel evaluationin each of integration periods.

The maximal energy window calculation unit compares the calculatedenergy output from the sliding multipath energy window calculationunits, selects the energy having maximal value and the phase of thelocal pilot PN code which corresponds to the energy. This result is sentto the initial synchronization confirmation unit (acquisitionconfirmation).

The initial synchronization confirmation unit may judges initialsynchronization using threshold judgement method for one time (thethreshold is determined as desired, in general, the threshold is larger1/10 than the total energy of the input signals). If the maximal energyis larger than the threshold, the initial synchronization process orcell search process is valid. Thus, the phase of local pilot PN code,which corresponds to the energy window having maximal value, is the bestPN code phase k_(opt). Otherwise, this initial synchronization processor cell search process is fail. Naturally, the threshold judgementmethod can be performed more times for confirming the initialsynchronization.

Next, the phase of the local PN code generation unit in a receiver isshifted to the best PN code phase k_(opt). After that, the process ofinitial synchronization or cell search is completed.

Since evaluations of S multipath energy windows can be obtained in eachof integration periods, the local cell PN code generation unit or theadjacent cell PN code generation unit as shown in FIG. 1 jumps S phasesbackward or forward after completing integration once.

Next, the time taken for initial synchronization is calculation. Sincethe local PN sequence has M×P possible PN code phase, the presentembodiment uses S parallels calculation units, therefore, S phases canbe searched within T_(d)=NT_(c) integration, period. Thus, the timetaken for initial synchronization is:

$\begin{matrix}{T_{acq} = {\frac{M \times P}{S}{NT}_{c}}} & (7)\end{matrix}$

The principle of adjacent cell search is similar with that of initialsynchronization, therefore, the process of adjacent cell search can beimplement in a maser of time division multiplexing. After initialsynchronization, the phase difference between the pilot sequence of theadjacent cells and the pilot sequence of the local cell and, the searchrange can be obtained by receiving the information broadcast from thelocal base station. The adjacent cell search can be performed byreplacing the local pilot sequence and whole search range used ininitial synchronization process with the local pilot sequence and searchrange of adjacent cells. FIG. 1 shows this time division multiplexingmanner.

Similarly, the time taken for adjacent cell search is calculated asfollows:

$\begin{matrix}{T_{srch} = {\frac{M \times P_{srch}}{S}{NT}_{c}}} & (8)\end{matrix}$wherein P_(srch) is the area to be searched (T_(c) as time unit). Aftersearching, the obtained maximal multipath energy window is reported tothe base station to determined whether or not hand-off ormacro-diversity shall be performed.

The implement of the present invention is described with a mobileterminal in CDMA 2000-1x system as an example. The downstream channel inCDMA 2000-1x system includes continuously transmitting pilot channelsused for extracting timing, initial synchronization, cell search andcoherent demodulation etc. In this system, the spreading chip rate is1.2288 Mcps, chip interval is T_(c)=1/1.2288 ms, the pilot channel PNcode is a pseudo random sequence with length N=2¹⁵.

If the receiver employs 4 times the chip sampling rate, that is, M=4,each integration period is 256 T_(c) and the maximal initialsynchronization time required for the system is T_(acq) _(—) _(max), thenumber of the parallel correlators is then:

$S > {\left\lceil \frac{4 \times 2^{15} \times 256\; T_{c}}{T_{acq\_ max}} \right\rceil + 1}$In fact, it may take T_(acq) _(—) _(max)<0.5 s, then, the number of theparallel correlators is S=64

Since CDMA 2000-1x system has a low spreading chip rate, the complexityassociated with hardware can be reduced with a manner of time divisionmultiplexing. In this example, each of physical correlators multiplexes32 times. Therefore, it needs 4 correlators in plural form. Thethreshold used for judging interference paths is set to 1/32 the energyof received signals. The threshold used for initial synchronization ofmaximal multipath energy window is set to 1/16 the energy of receivedsignals.

This example can apply to a vehicle mobile station in CDMA2000-1xcellular mobile communication system fitting Standard 3GPP2 Release A.The spread spectrum receiving part in the mobile station can beimplemented by, for example, a XC4085x1a FPGA chip, a product of Xilinxcompany. The spread spectrum receiver to which the apparatus accordingto the invention is applied can provide excellent stability in thecircumstances of vehicle mobile terminals.

Industry Practicability

The initial synchronization method based on multipath energy windowaccording to the invention can maximize the multipath time-delaydistribution energy, can overcome the non-determinacy of the system dueto the processing of single path signals in conventional initialsynchronization method. Therefore, the apparatus according to theinvention is applicable to mobile cellular communication system in thecircumstances of multipath fading channel. The apparatus of theinvention employs a sliding multipath energy window calculation methodand is facilitated to implement. The apparatus of the inventionintegrates the initial synchronization and adjacent cell search of aCDMA receiver in the manner of time division multiplexing (TDM), thecomplexity of the hardware used in a system is therefore greatlyreduced. It takes a shorter time for initial synchronization andadjacent cell search since the calculation is operated in parallel.

1. An apparatus for initial synchronization and cell search in CDMAsystems based on a multipath energy window comprises: a plurality of tapdelay lines, in which the number S of the tap delay lines is determinedby the minimal time required for initial synchronization and cellsearch, for sampling baseband signals r(t) with a rate T_(c)/M, sendingthe S tap outputs from the S tap delay lines to S parallel correlators,where T_(c)/M is a fraction sampling interval, T_(c) is the chip rate ofthe CDMA system and M is the oversampling rate; each of the S parallelcorrelators performs channel evaluation of a single path in oneintegrating period NT_(C), clears the respective multiplyingaccumulation units of each correllator after sending the evaluatedresults to respective amplitude judging units; a plurality of amplitudejudging units for judging whether the square value of the channelevaluation amplitude obtained from respective correlators is larger thana threshold which is used for judging a signal arrival path as a pureinterference path when the evaluation amplitude is lower than thethreshold, sending the evaluated amplitude with which the pureinterference paths are replaced with a zero value to a buffer, and thenproviding the evaluated amplitudes to a sliding multipath energy windowcalculation unit one by one through a parallel-series conversion unit; asliding multipath energy window calculation unit for calculating Soutputs from sliding multipath energy windows for the S channelevaluations obtained in every integration interval; and a maximal energywindow calculation unit for comparing every value output from thesliding multipath energy windows and selecting the amplitude havingmaximal value and the phase of the local pilot PN code which correspondsto the amplitude.
 2. The apparatus for initial synchronization and cellsearch in CDMA systems based on a multipath energy window according toclaim 1, wherein each of the S parallel correlators performs channelevaluation integration operation as follows:${{{\overset{\_}{c}}_{n,m}(k)} = {\frac{1}{{NE}_{c}}{\int_{0}^{{NT}_{c}}{{{r\left( {t - {nT}_{c} - {{mT}_{c}/M}} \right)} \cdot {s_{0}^{*}\left( {t - {{kT}_{c}/M}} \right)}}{\mathbb{d}t}}}}},{m = 0},1,{{\cdots\mspace{11mu} M} - 1}$where k is a possible phase parameter of the local pilot PN sequence, Mis a real number, n denotes the nth arrival path, and s₀ denotes a localPN Ser. No.; wherein, for the process of-initial synchronization, theintegration operation uses the PN code output from a local cell PN codegeneration unit; and for the process of cell search, the integrationoperation uses the PN code output from an adjacent cell PN codegeneration units.
 3. The apparatus for initial synchronization and cellsearch in CDMA systems based on a multipath energy window according toclaim 2, wherein the sliding multipath energy window calculation unitcalculates S outputs from sliding multipath energy windows with respectto the S channel evaluations in each integration interval as follows:E _(win)(k+1)=E _(win)(k)−| c _(L) _(2,) _(M-1)(k)|² +| c _(L) ₁_(,0)(k+1)|^(2.)
 4. The apparatus for initial synchronization and cellsearch in CDMA systems based on a multipath energy window according toclaim 1, further comprises an initial synchronization confirmation unitfor judging the maximal value from the outputs from maximal energywindow calculation unit and the local pilot PN code phase associatedwith the maximal value; wherein if the maximal energy is larger than athreshold, the process of initial synchronization or cell search isvalid, and the phase of local pilot PN code associated with the energywindow having maximal value is the best PN code phase k_(opt),otherwise, the process of initial synchronization or cell search fails.5. The apparatus for initial synchronization and cell search in CDMAsystems based on a multipath energy window according to claim 1, whereinthe local PN code generation unit is shifted to the best PN code phasek_(opt) if the process of initial synchronization process or cell searchis valid.
 6. The apparatus for initial synchronization and cell searchin CDMA systems based on a multipath energy window according to claim 1,wherein the initial synchronization confirmation unit judges initialsynchronization with a threshold judgement one time.
 7. The apparatusfor initial synchronization and cell search in CDMA systems based on amultipath energy window according to claim 1, wherein the initialsynchronization confirmation unit only needs to calculate the maximalvalue of the multipath energy window at k=k_(opt), one or more times,and the initial synchronization is successful if the corresponding oneor more results are larger than the threshold.
 8. The apparatus forinitial synchronization and cell search in CDMA systems based on amultipath energy window according to claim 1, wherein the adjacent cellsearch is implemented in the manner of time division multiplexing.